1. Field of the Invention
The present invention relates to the intraprediction of an image and, more particularly, to a method and apparatus for image intraprediction encoding/decoding to improve image compression efficiency.
2. Description of the Related Art
In image compression standards such as moving picture expert group (MPEG)-1, MPEG-2, MPEG-4 Visual, H.261, H.263, and H.264 standards, a picture is generally divided into macroblocks for video encoding. After each of the macroblocks is encoded in all interprediction and intraprediction encoding modes, bit rates required for encoding the macroblock and rate-distortion (RD) costs for the encoding modes are compared. Then an appropriate encoding mode is selected according to the results of the comparison and the macroblock is encoded in the selected encoding mode.
In intraprediction, instead of referring to reference pictures, a prediction value of a macroblock to be encoded is calculated using a pixel value of a pixel that is spatially adjacent to the macroblock to be encoded and a difference between the prediction value and the pixel value is encoded when encoding macroblocks of a current picture.
FIG. 1 illustrates the use of previous macroblocks for the intraprediction of a current macroblock a according to the prior art.
Referring to FIG. 1, previous macroblocks a1, a2, a3, and a4 are used for the intraprediction of the current macroblock a5. According to a raster scan scheme, macroblocks included in a picture are scanned left-to-right and top-to-bottom. Thus, the previous macroblocks a1, a2, a3, and a4 are scanned and encoded before the current macroblock a5. Because they are not encoded, macroblocks marked with an X cannot be used for predictive encoding of the current macroblock a5. Because they have low correlation with the current macroblock a5, macroblocks marked with O are not used for predictive encoding of the current macroblock a5. After being transformed using a discrete cosine transform (DCT) and quantized, the previous macroblocks a1, a2, a3, and a4 are inversely quantized and inversely transformed using a DCT and then reconstructed.
FIG. 2 is a reference diagram indicating adjacent pixels used in 4×4 intra modes of the H.264 standard according to the prior art.
Referring to FIG. 2, lower-case letters a through p indicate pixels of a 4×4 block to be predicted, and upper-case letters A through M located above and to the left of the 4×4 block indicate adjacent samples or pixels that are required for intraprediction of the 4×4 block and have already been encoded and reconstructed.
FIG. 3 illustrates 4×4 intra modes used in the H.264 standard according to the prior art.
Referring to FIG. 3, there are nine 4×4 intra modes, i.e., a direct current (DC) mode, a vertical mode, a horizontal mode, a diagonal down-left mode, a diagonal down-right mode, a vertical left mode, a vertical right mode, a horizontal up mode, and a horizontal down mode. In the 4×4 intra modes, pixel values of pixels a through p are predicted from pixels A through M of adjacent macroblocks. As illustrated in FIG. 3, in the 4×4 intra modes, adjacent pixels in a frame including a block to be intrapredicted are used as reference pixels. As such, in an intraprediction method according to the prior art, a prediction value of a macroblock to be encoded is calculated using pixel values of pixels that are spatially adjacent to the macroblock to be encoded.
FIGS. 4A and 4B illustrate processing orders of 4×4 blocks included in a macroblock in a 4×4 intra mode according to prior art.
According to a processing order of 4×4 blocks in the H.264 standard, 4×4 blocks included in a single macroblock are processed in numerical order based on the numbers indicated in the blocks of FIG. 4A, i.e., left-to-right and top-to-bottom.
Since a current 4×4 block can be intrapredicted using only pixel values of pixels located above and to the left of the current 4×4 block as illustrated in FIG. 2, an improved encoding method is required to improve encoding efficiency.